Zeolitic materials having chabazite (CHA) framework structure are widely used in important actual technical areas such as in the automotive industry where the materials are employed as catalysts. The reduction of nitrogen oxides with ammonia to form nitrogen and H2O can be catalyzed by metal-promoted zeolites to take place preferentially to the oxidation of ammonia by the oxygen or to the formation of undesirable side products such as N2O, hence the process is often referred to as the “selective” catalytic reduction (“SCR”) of nitrogen oxides, and is sometimes referred to herein simply as the “SCR” process. The catalysts employed in the SCR process ideally should be able to retain good catalytic activity over the wide range of temperature conditions of use, for example, 200° C. to 600° C. or higher, under hydrothermal conditions and in the presence of sulfur compounds. High temperature and hydrothermal conditions are often encountered in practice, such as during the regeneration of the catalyzed soot filter, a component necessary for the aftertreatment of exhaust off-gas. Thus, these materials are of high economical and ecological interest. Due to the said technical areas and the resulting need of high amounts of the materials, there is an increasing demand for efficient processes for the preparation of these materials.
Molecular sieves are classified by the Structure Commission of the International Zeolite Association according to the rules of the IUPAC Commission on Zeolite Nomenclature. According to this classification, framework-type zeolites and other crystalline microporous molecular sieves, for which a structure has been established, are assigned a three letter code and are described in the Atlas of Zeolite Framework Types, 5th edition, Elsevier, London, England (2001). Chabazite is one of the molecular sieves for which a structure has been established, and the material of this framework-type is designated as CHA. Zeolitic materials as used herein are defined as metallosilicate frameworks including aluminosilicates, borosilicates and gallosilicates. It does not include the MeAPSO, APSO, or AIPO family of materials.
Chabazite is a zeolite which occurs in nature and also has synthetic forms. Synthetic forms are described in “Zeolite Molecular Sieves” by Breck (1973). The structure of Chabazite is described in “Atlas of Zeolite Structure Types” by Meier and Olson (1978). The Chabazite structure has been designated with the structure code, “CHA”.
Natural Chabazite exists in nature and has a SiO2:Al2O3 typically less than 10. Synthetic forms of this low SiO2:Al2O3 range include zeolite “K-G”, zeolite D and zeolite R. Zeolite “K-G” is reported by Barrer et al. in J. Chem. Soc., 1956, p 2892-. Zeolite D is reported in British patent number 868,846. Zeolite R is reported in U.S. Pat. No. 3,030,181.
Synthesis of high-silica Chabazite (>10 SiO2:Al2O3) is reported in U.S. Pat. No. 4,544,538, U.S. Pat. No. 6,709,644 and US 2003/0176751 A1.
U.S. Pat. No. 6,709,644 discloses a high-silica CuChabazite (SSZ-62) with small crystal size (<0.5 microns) with application in SCR of NOx.
WO 2008/106519 discloses a catalyst comprising: a zeolite having the CHA crystal structure and a mole ratio of silica to alumina greater than 15 and an atomic ratio of copper to aluminum exceeding 0.25. The catalyst is prepared via copper exchanging NH4+-form CHA with copper sulfate or copper acetate. Catalytic activity is largely retained after hydrothermal aging at 850° C. for 6 hours.
WO 2008/132452 discloses a number of zeolite materials, including CuSSZ-13, that can be loaded with iron and/or copper. Catalytic activity is largely retained after hydrothermal aging of CuSSZ-13 at 900° C. for 1 hour. Although no specific mention of Na levels appears it is stated than an ammonium exchange is employed prior to the Cu exchange to remove Na.
WO 2008/118434 indicates that a CuSSZ-13 (15 to 60 SiO2:Al2O3) material that can retain at least 80% of its surface area and micropore volume after hydrothermal aging at 900° C. in 10% steam for 1 to 16 hours would be suitable for application in SCR. Example 3 indicates that an ammonium exchange is carried out to remove residual Na. Additionally, a comparison of medium-sized crystals to large-sized crystals of SAPO-34 indicated improved stability for the larger crystals.
In all cases Na is first removed by ammonium exchange prior to the introduction of Cu. The resultant Na content is not disclosed. In table 8 of U.S. Pat. No. 4,544,538 Na contents of >0.5% Na2O are reported for examples 2 through 5 following ammonium exchange. Prior to ammonium exchange the Chabazites prepared with alkali metal hydroxides in the synthesis gel would be expected to contain >0.5 wt % Na2O.
The state of the art preparation of a Cu-Chabazite is described by the following key steps:                1. Crystallization of a alkali metal/SDA containing chabazite and separation from the synthesis gel        2. Drying and calcination to remove the SDA leading to the H—Na(alkali) form of Chabazite        3. Ammonium exchange to remove alkali metals        4. Copper exchange to introduce Cu        